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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

NUNATAK AND CASCADING GLACIERS                   143
lips of the hanging valleys by the recession of the trunk glaciers. Applying this explanation specifically to the Cascading Glacier of Nunatak Fiord, the evidence is conclusive that the entire fiord, up to a level much higher than the point where Cascading Glacier emerges from its valley, was formerly filled with ice which moved vigorously westward. The deposits left by this expanded Nunatak Glacier, and the grooves and flutings which it wore in the rock of the mountain sides are plainly visible throughout the fiord. That this greatly-expanded glacier flowed vigorously, and for a long period of time, is proved by the series of perfect hanging valleys perched high above the water surface on both sides of the main fiord. The inference is, therefore, warranted that before Nunatak Glacier reached this expanded condition there was a main valley whose bottom was at least as high as the bottoms of the hanging valleys, and that while the lateral valleys have been worn by glacial erosion to some extent, the main valley has been worn much faster and much deeper.
During the ice-flood condition of the Nunatak Glacier these lateral glaciers presumably entered the main glacier with their surface levels approximately accordant with the surface of the main glacier. Such a condition is now observed in tributaries of the larger glaciers far back in the mountains. In these cases, however, there is usually a step or descent in the glacier surface just above where the tributary joins the main glacier. Whether this condition is due to a moderate recession of the main glacier, or whether it is normal to the flooded stage, cannot be stated on the basis of direct observation, but by inference it seems probable that in the stage of highest ice flood the tributary glacier surface would be approximately accordant with that of the main glacier and that the step would be absent. During the flood stage the tributary ice streams contribute a greater or less amount of ice to the main glacier according to their size; but in the great majority of cases these tributaries are so small that as soon as they join the main stream they are at once dominated by the powerful thrust of the main glacier. That this was true of the Cascading Glacier is clearly proved by the huge parallel rock grooves on the mountain slope beneath it. These were without question eroded by the vigorous westward-moving Nunatak Glacier, and there is no sign of grooving resulting from the incoming of the Cascading Glacier.
When the main glacier begins to wane, its thickness decreases while, at the same time, the front recedes up-stream, and thus ultimately, because of thinning, the fact of discordance between the bettom levels of the tributary and main valleys become apparent by the development of a step where the tributary emerges to join the main ice stream. With continued lowering of the main glacier the length of this step increases, and we may ultimately have a cascading glacier extending from the lip of its valley down almost to the base of the steepened slope beneath the hanging valley. This stage in the development of cascading glaciers is illustrated by a tributary on the south side of the Nunatak Glacier about a mile east of the Cascading Glacier; It descends from a valley of about the same height as that of the Cascading Glacier, but its cascading front extends clear down to the glacier surface. Scores of instances of similar condition are found in the Alaskan region. The next step consists in the discontinuation of the tributary end, and then we have the production of the typical cascading glacier, of which the one under consideration is a perfect example.
Once the cascading glacier is disconnected from the main ice stream to which it was formerly tributary, its recession is relatively rapid. This is due first to the steepness